Method of screen printing

ABSTRACT

The material to be applied by a screen printing technique is mixed with a thermoplastic vehicle which is solid at room temperature and melts at a predetermined elevated temperature. The material is formed into a slab which is then pressed against a screen surface. The screen is heated enough to melt a portion of the slab and the molten material is then forced through the screen by a squeegee.

iimted States Patent 1151 3,656,48 Duncan 1451 Apr. 18, 1972 [54] METHOD OF SCREEN PRINTING 987,169 3/1911 Reimann et al ..4o1/2 1,740,285 12/1929 Cdrr'ell ..101/31 [72] Invent Gwge Duncan 2,057,696 10/1936 Sherman ..101/31 x [73] Assignee: General Motors Corporation, Detroit, 2,895,412 7/1959 Reed 101/126 X Mich. 3,082,688 3/1963 Shennan ..101/126 [22] Filed: June 15, 1970 3,133,484 5/1964 Wright ..10l/ESD PP 46,115 I Primary Examiner-William B. Penn Attorney-Jean L. Carpenter, Paul Fitzpatrick and Warren D. 52 us. c1 ..101/129, 101/126 [51] lnt.Cl. ..B41l 13/00 B4 1m H12 58 Field ol'Search ..101/129,126,123,115,114, 1 ABSTRACT 101/311 27; 401/1} 2 The material to be applied by a screen printing technique is mixed with a thermoplastic vehicle which is solid at room tem- [56] Rem-wees cued perature and melts at a predetermined elevated temperature.- UNITED STATES PATENTS The material is formed into a slab which is then pressed against a screen surface. The screen is heated enough to melt a 2,339,423 H1944 Pollard 101/126 o tio of the slab and the molten material is then forced 2,731,912 1/1956 Welsh ....101/129 h h th screen byasqueegee. 3,235,706 2/1966 Neumann et al.... ....40l/1 X 2,845,859 8/1958 Gattuso ..101/l26 1Claims,21i)rawing Figures PATENTEBAPR 181972 3, 656.428

A I la INVEN'IOR f BY Gary/1.0mm:

METHOD OF SCREEN PRINTING This invention relates to a method of screen printing and more particularly to a method of screen printing using a printing medium which is in a solid state at normal room temperature.

It is known to use a printing medium which is in a solid state at room temperature and which melts at a higher temperature. The advantage of such a medium is that upon being applied to a substrate at room temperature, it congeals and may be readily handled or processed prior to being fixed or fired. The usual practice in the use of such material is to melt the printing medium and store it in a reservoir held at an elevated temperature and then discharge the molten material onto a heated screen where the medium is thenapplied to a substrate in the conventional manner.

It is a general object of this invention to provide a method of printing with a thermofiuid medium which does not require the use of a heated reservoir.

It is a further object of the invention to provide a method of screen printing in which the printing medium is in a solid state until application of the medium to the screen.

The invention is carried out by casting slabs of material from a thermofluid composition, providing heated screens carrying a desired pattern configuration, placing a solid slab in contact with the heated screen to melt a portion of the slab and finally, to force a portion of the molten material. through the screen onto a substrate.

The above and other advantages of the invention will become more apparent form the following description taken in conjunction with the accompanying drawings wherein like reference numerals refer to like parts and wherein:

FIGS. 1 and 2 are cross-sectional elevational views of screen printing apparatus illustrating the practice of the subject invention.

Thermofluid screen printing medium is available, for example, for the purpose of printing labels on glass bottles. The medium comprises a mixture of a colored pigment, a glass binder and a vehicle comprising a wax-like thermoplastic material which is solid at room temperature and melts at about 80 C. Similar materials are available for screen printing electrical conductors and resistors for the formation of thick film printed circuits. These latter media comprise a silver paladium mixture with a glass binder and a thermoplastic vehicle. The media is generally available in small hard fragments. It is well known to apply such thermofluid printing media to substrates by melting the thermofluid material, holding it in a reservoir and then applying it to a heated screen and forcing the molten material through the screen by a squeegee. The screens are heated directly or indirectly. The directly heated screens are heated by passing electrical current through the screens and heat is generated therein by the electrical resistance of the screen. The indirectly heated screens are heated externally as by infrared lamps.

According to the present invention and referring to FIGS. 1 and 2 of the drawings, the thermofluid media is first melted and then cast into a slab 10. A slab is placed in a squeegee holder 12 which comprises a pair of vertical walls 14 spaced to define a feed channel 15 which receives the slab 10. The squeegee holder carries at its lower end a pair of squeegees 16, one on either side of the solid slab 10. It is the usual practice to form a squeegee 16 of rubber, metal, Teflon or the like. A screen 18 comprises a masked portion 20 and an open mesh pattern portion 22 which is shaped in the configuration of the desired pattern of deposit to be laid on the substrate 24. The screen 18, of course, is held adjacent to the substrate and is either in contact with or slightly spaced therefrom. The screen is heated as shown in the drawings by infrared lamps 26. As explained above, however, the screen may be heated directly by passing electrical current therethrough.

In operation, the screen 18 is heated above themelting point of the slab 10 of thermofluid printing medium or about 90 C. The slab 10 is urged against the screen 18 by its own weight. If additional pressure is desired, then the slab may be spring-loaded, a weight 28 may be added to the top of the slab or a second slab may be stacked in the holder 12 atop the slab 10. When the edge of the slab 10 engages the screen 18, a portion of the slab melts to form a molten pool 30. Then as the slab holder 12 traverses the screen 18, the molten media is forced through the screen by a squeegee 16 onto the substrate, newly molten material from the slab replenishes that material which is used and the slab 10 continuously feeds down to maintain contact with the screen. By using a squeegee on each side of the slab 10 it is possible to print during both forward and reverse motion of the squeegee holderl2. The amount of material deposit on the screen is a.function of the screen temperature and the traverse speed of the squeegee. Hence, it is possible to deposit very close to the necessary amount of printing medium.

While the method of this invention was specifically intended for the deposition of conductive and resistive thennofuid materials for use in thick film microelectronics, the method is adaptable to general usage.

The embodiment of the invention described is for the purpose of illustration and the scope of the invention is intended to be limited only by the following claims:

1. The method of screen printing a pattern onto a substrate comprising casting a slab of thermoplastic material which is solid at room temperature and which becomes molten at a predetermined temperature,

providing a screen having the desired pattern configuration adjacent the substrate,

heating the screen to a temperature above the predetermined temperature,

melting a portion of the slab onto the heated screen by moving the slab along the screen in contact therewith,

and forcing the molten material through the screen by passing a squeegee positioned behind the slab across the screen surface conjointly with slab movement to form on the substrate a pattern according to the screen configuration. 

